XCTExecute returning values and throwing errors

[adam-fowler]

This is a breaking change:
HBApplication.XCTExecute will now return the value the supplied closure returns. This simplifies testing code where you need a value from one response to use in the next.

var optionalHeader: String?
app.XCTExecute(uri: "/", method: .GET) { response in
    optionalHeader = response.header["my-header"].first
}
let header = try XCTUnwrap(optionalHeader)

now becomes

let header = try app.XCTExecute(uri: "/", method: .GET) { response in
    return response.header["my-header"].first
}

which is much cleaner.

To get this to work though requires XCTExecute to throw errors