close connections after request is processed

[artemredkin]

Since we don't have a connection pool, we need to close channel after request is processed, this PR addresses #33

[weissi]

Looks good to me (until we get the connection pool; CC @adtrevor)

[weissi]

@artemredkin tests failed:

Test Case 'SwiftHTTPTests.testHTTPPartsHandler' started at 2019-05-14 16:04:08.514 /code/Tests/NIOHTTPClientTests/SwiftNIOHTTPTests.swift:58: error: SwiftHTTPTests.testHTTPPartsHandler : XCTAssertEqual failed: ("head(HTTPRequestHead{method: GET, uri: "/get", version: HTTP/1.1, headers: [("X-Test-Header", "X-Test-Value"), ("Host", "localhost"), ("Content-Length", "4")] })") is not equal to ("head(HTTPRequestHead{method: GET, uri: "/get", version: HTTP/1.1, headers: [("X-Test-Header", "X-Test-Value"), ("Host", "localhost"), ("Connection", "close"), ("Content-Length", "4")] })") - Test Case 'SwiftHTTPTests.testHTTPPartsHandler' failed (0.002 seconds) 

expected is missing the connection: close header

[artemredkin]

Test fixed