TwitterOAuthSignatureBaseString.swift

private func oAuthSignatureBaseString(httpMethod: String,
                                      baseURLString: String,
                                      parameters: [URLQueryItem]) -> String {
    var parameterComponents: [String] = []
    for parameter in parameters {
        let name = parameter.name.oAuthURLEncodedString
        let value = parameter.value?.oAuthURLEncodedString ?? ""
        parameterComponents.append("\(name)=\(value)")
    }
    let parameterString = parameterComponents.sorted().joined(separator: "&")
    return httpMethod + "&" +
        baseURLString.oAuthURLEncodedString + "&" +
        parameterString.oAuthURLEncodedString
}