Only add Content-Type if not specified in headers. Fixes #20046 (#20234)

* Only add Content-Type if not specified in headers. Fixes #20046

* Update documentation to indicate body_format will not override Content-Type if specified in headers

lib/ansible/modules/network/basics/uri.py

@@ -65,6 +65,8 @@ options:
     description:
       - The serialization format of the body. When set to json, encodes the
         body argument, if needed, and automatically sets the Content-Type header accordingly.
+        As of C(2.3) it is possible to override the `Content-Type` header, when
+        set to json via the I(headers) option.
     required: false
     choices: [ "raw", "json" ]
     default: raw
@@ -134,7 +136,9 @@ options:
     default: null
   headers:
     description:
-        - Add custom HTTP headers to a request in the format of a YAML hash
+        - Add custom HTTP headers to a request in the format of a YAML hash. As
+          of C(2.3) supplying C(Content-Type) here will override the header
+          generated by supplying C(json) for I(body_format).
     required: false
     default: null
     version_added: '2.1'
@@ -399,7 +403,9 @@ def main():
         # Encode the body unless its a string, then assume it is pre-formatted JSON
         if not isinstance(body, basestring):
             body = json.dumps(body)
-        dict_headers['Content-Type'] = 'application/json'
+        lower_header_keys = [key.lower() for key in dict_headers]
+        if 'content-type' not in lower_header_keys:
+             dict_headers['Content-Type'] = 'application/json'
 
     # Grab all the http headers. Need this hack since passing multi-values is
     # currently a bit ugly. (e.g. headers='{"Content-Type":"application/json"}')