TwitterOAuthSignature.swift

private func oAuthSignature(httpMethod: String,
                            baseURLString: String,
                            parameters: [URLQueryItem],
                            consumerSecret: String,
                            oAuthTokenSecret: String? = nil) -> String {
    let signatureBaseString = oAuthSignatureBaseString(httpMethod: httpMethod,
                                                       baseURLString: baseURLString,
                                                       parameters: parameters)

    let signingKey = oAuthSigningKey(consumerSecret: consumerSecret,
                                     oAuthTokenSecret: oAuthTokenSecret)

    return signatureBaseString.hmacSHA1Hash(key: signingKey)
}