Add support for retrieving headers in canonical form

Sources/NIOHTTP1/HTTPTypes.swift

@@ -192,6 +192,23 @@ public struct HTTPHeaders : Sequence, CustomStringConvertible {
     public func makeIterator() -> AnyIterator<(name: String, value: String)> {
         return AnyIterator(HTTPHeadersIterator(wrapping: storage.makeIterator()))
     }
+
+    /// Retrieves the header values for the given header field in "canonical form": that is,
+    /// splitting them on commas as extensively as possible such that multiple values received on the
+    /// one line are returned as separate entries. Also respects the fact that Set-Cookie should not
+    /// be split in this way.
+    public func getCanonicalForm(_ name: String) -> [String] {
+        // It's not safe to split Set-Cookie on comma.
+        let queryName = name.lowercased()
+        if queryName == "set-cookie" {
+            return self[name]
+        }
+
+        if let result = storage[queryName] {
+            return result.map { tuple in tuple.1.split(separator: ",").map { substring in String(substring).trimmingCharacters(in: .whitespaces) } }.reduce([], +)
+        }
+        return []
+    }
 }
 
 extension HTTPHeaders : Equatable {

Tests/NIOHTTP1Tests/HTTPHeadersTest+XCTest.swift

@@ -28,6 +28,9 @@ extension HTTPHeadersTest {
       return [
                 ("testCasePreservedButInsensitiveLookup", testCasePreservedButInsensitiveLookup),
                 ("testWriteHeadersSeparately", testWriteHeadersSeparately),
+                ("testRevealHeadersSeparately", testRevealHeadersSeparately),
+                ("testSubscriptDoesntSplitHeaders", testSubscriptDoesntSplitHeaders),
+                ("testCanonicalisationDoesntHappenForSetCookie", testCanonicalisationDoesntHappenForSetCookie),
            ]
    }
 }

Tests/NIOHTTP1Tests/HTTPHeadersTest.swift

@@ -70,4 +70,43 @@ class HTTPHeadersTest : XCTestCase {
         XCTAssertTrue(writtenBytes.contains("set-cookie: foo=bar\r\n"))
         XCTAssertTrue(writtenBytes.contains("set-cookie: buz=cux\r\n"))
     }
+
+    func testRevealHeadersSeparately() {
+        let originalHeaders = [ ("User-Agent", "1"),
+                                ("host", "2"),
+                                ("X-SOMETHING", "3, 4"),
+                                ("X-Something", "5")]
+
+        let headers = HTTPHeaders(originalHeaders)
+        XCTAssertEqual(headers.getCanonicalForm("user-agent"), ["1"])
+        XCTAssertEqual(headers.getCanonicalForm("host"), ["2"])
+        XCTAssertEqual(headers.getCanonicalForm("x-something"), ["3", "4", "5"])
+        XCTAssertEqual(headers.getCanonicalForm("foo"), [])
+    }
+
+    func testSubscriptDoesntSplitHeaders() {
+        let originalHeaders = [ ("User-Agent", "1"),
+                                ("host", "2"),
+                                ("X-SOMETHING", "3, 4"),
+                                ("X-Something", "5")]
+
+        let headers = HTTPHeaders(originalHeaders)
+        XCTAssertEqual(headers["user-agent"], ["1"])
+        XCTAssertEqual(headers["host"], ["2"])
+        XCTAssertEqual(headers["x-something"], ["3, 4", "5"])
+        XCTAssertEqual(headers["foo"], [])
+    }
+
+    func testCanonicalisationDoesntHappenForSetCookie() {
+        let originalHeaders = [ ("User-Agent", "1"),
+                                ("host", "2"),
+                                ("Set-Cookie", "foo=bar; expires=Sun, 17-Mar-2013 13:49:50 GMT"),
+                                ("Set-Cookie", "buz=cux; expires=Fri, 13 Oct 2017 21:21:41 GMT")]
+
+        let headers = HTTPHeaders(originalHeaders)
+        XCTAssertEqual(headers.getCanonicalForm("user-agent"), ["1"])
+        XCTAssertEqual(headers.getCanonicalForm("host"), ["2"])
+        XCTAssertEqual(headers.getCanonicalForm("set-cookie"), ["foo=bar; expires=Sun, 17-Mar-2013 13:49:50 GMT",
+                                                                "buz=cux; expires=Fri, 13 Oct 2017 21:21:41 GMT"])
+    }
 }