urequests: Actually implement user-defined headers support.

urequests/urequests.py

@@ -57,6 +57,12 @@ def request(method, url, data=None, json=None, headers={}, stream=None):
     s.write(b"%s /%s HTTP/1.0\r\n" % (method, path))
     if not "Host" in headers:
         s.write(b"Host: %s\r\n" % host)
+    # Iterate over keys to avoid tuple alloc
+    for k in headers:
+        s.write(k)
+        s.write(b": ")
+        s.write(headers[k])
+        s.write(b"\r\n")
     if json is not None:
         assert data is None
         import ujson