Add precondition assertions in scheme, host matchers

[Liquidsoul]

Also added an explanation of which part it is in an example URL.

Checklist

• I've checked that all new and existing tests pass
• I've updated the documentation if necessary
• I've added an entry in the CHANGELOG to credit myself

Description

Add preconditions so that user are better informed of why their requests are not stubbed.
This also adds information of what is a scheme, host or path in the documentation of each matcher.

Motivation and Context

This will help users see why their matchers setup does not work (see #247).

[AliSoftware]

Seems like contains was not available in Swift 2.3 🙄 Maybe add an extension String in the #if !swift(>=3.0) block at the top of this file to add support for it?

OHHTTPStubs/OHHTTPStubs/Sources/Swift/OHHTTPStubsSwift.swift

Lines 30 to 55 in 5956336

	#if !swift(>=3.0)
	extension OHHTTPStubs {
	private class func stubRequests(passingTest passingTest: OHHTTPStubsTestBlock, withStubResponse: OHHTTPStubsResponseBlock) -> OHHTTPStubsDescriptor {
	return stubRequestsPassingTest(passingTest, withStubResponse: withStubResponse)
	}
	}
	extension NSURLRequest {
	var httpMethod: String? { return HTTPMethod }
	var url: NSURL? { return URL }
	}
	extension NSURLComponents {
	private convenience init?(url: NSURL, resolvingAgainstBaseURL: Bool) {
	self.init(URL: url, resolvingAgainstBaseURL: resolvingAgainstBaseURL)
	}
	}
	private typealias URLRequest = NSURLRequest
	extension URLRequest {
	private func value(forHTTPHeaderField key: String) -> String? {
	return valueForHTTPHeaderField(key)
	}
	}
	#endif

[Liquidsoul]

Yup saw that, I'll look into it later as soon as I can 😉