CONNECT method should not have request body (#637)

* CONNECT method has no request body

### Motivation:

[rfc7231](https://tools.ietf.org/html/rfc7231#section-4.3.6) said:
> A payload within a `CONNECT` request message has no defined semantics;
   sending a payload body on a `CONNECT` request might cause some existing
   implementations to reject the request.

### Modifications:

Change `HTTPMethod.CONNECT.hasRequestBody` to returns `.no`.

### Result:

`HTTPRequestEncoder` does not generate chunked body for `CONNECT` method.

* Change `HTTPMethod.CONNECT.hasRequestBody` to returns `.unlikely`

Sources/NIOHTTP1/HTTPTypes.swift

@@ -959,9 +959,9 @@ public enum HTTPMethod: Equatable {
         switch self {
         case .HEAD, .DELETE, .TRACE:
             return .no
-        case .POST, .PUT, .CONNECT, .PATCH:
+        case .POST, .PUT, .PATCH:
             return .yes
-        case .GET, .OPTIONS:
+        case .GET, .CONNECT, .OPTIONS:
             fallthrough
         default:
             return .unlikely

Tests/NIOHTTP1Tests/HTTPRequestEncoderTest+XCTest.swift

@@ -33,6 +33,7 @@ extension HTTPRequestEncoderTests {
                 ("testNoTransferEncodingHeadersForHEAD", testNoTransferEncodingHeadersForHEAD),
                 ("testNoChunkedEncodingForHTTP10", testNoChunkedEncodingForHTTP10),
                 ("testBody", testBody),
+                ("testCONNECT", testCONNECT),
            ]
    }
 }

Tests/NIOHTTP1Tests/HTTPRequestEncoderTest.swift

@@ -116,6 +116,24 @@ class HTTPRequestEncoderTests: XCTestCase {
         assertOutboundContainsOnly(channel, "")
     }
 
+    func testCONNECT() throws {
+        let channel = EmbeddedChannel()
+        defer {
+            XCTAssertEqual(.some(false), try? channel.finish())
+        }
+
+        let uri = "server.example.com:80"
+        try channel.pipeline.add(handler: HTTPRequestEncoder()).wait()
+        var request = HTTPRequestHead(version: HTTPVersion(major: 1, minor:1), method: .CONNECT, uri: uri)
+        request.headers.add(name: "Host", value: uri)
+
+        XCTAssertNoThrow(try channel.writeOutbound(HTTPClientRequestPart.head(request)))
+        XCTAssertNoThrow(try channel.writeOutbound(HTTPClientRequestPart.end(nil)))
+
+        assertOutboundContainsOnly(channel, "CONNECT \(uri) HTTP/1.1\r\nHost: \(uri)\r\n\r\n")
+        assertOutboundContainsOnly(channel, "")
+    }
+
     private func assertOutboundContainsOnly(_ channel: EmbeddedChannel, _ expected: String) {
         if case .some(.byteBuffer(let buffer)) = channel.readOutbound() {
             buffer.assertContainsOnly(expected)