Add assertion when using gRPC with URLSession (#138)

Add a debug assertion failure that will trigger when the
`URLSessionHTTPClient` is used with gRPC to make it more apparent as to
why some "successful" responses are still invalid.

For context, URLSession does not support trailers and is thus
incompatible with gRPC.

Libraries/Connect/Implementation/URLSessionHTTPClient.swift

@@ -45,6 +45,7 @@ open class URLSessionHTTPClient: NSObject, HTTPClientInterface {
         onMetrics: @Sendable @escaping (HTTPMetrics) -> Void,
         onResponse: @Sendable @escaping (HTTPResponse) -> Void
     ) -> Cancelable {
+        assert(!request.isGRPC, "URLSessionHTTPClient does not support gRPC, use NIOHTTPClient")
         let urlRequest = URLRequest(httpRequest: request)
         let task = self.session.dataTask(with: urlRequest) { data, urlResponse, error in
             if let httpURLResponse = urlResponse as? HTTPURLResponse {
@@ -93,6 +94,7 @@ open class URLSessionHTTPClient: NSObject, HTTPClientInterface {
     open func stream(
         request: HTTPRequest, responseCallbacks: ResponseCallbacks
     ) -> RequestCallbacks {
+        assert(!request.isGRPC, "URLSessionHTTPClient does not support gRPC, use NIOHTTPClient")
         let urlSessionStream = URLSessionStream(
             request: URLRequest(httpRequest: request),
             session: self.session,
@@ -196,6 +198,15 @@ extension Code {
     }
 }
 
+private extension HTTPRequest {
+    var isGRPC: Bool {
+        return self.headers[HeaderConstants.contentType]?.first.map { contentType in
+            return contentType.hasPrefix("application/grpc")
+                && !contentType.hasPrefix("application/grpc-web")
+        } ?? false
+    }
+}
+
 private extension URLRequest {
     init(httpRequest: HTTPRequest) {
         self.init(url: httpRequest.url)