Merge two approaches for GAM header on SA

- Ross approach put headers on both SA token generation request and API calls
- Jay approach appending to existing header if it exists

src/gam.py

@@ -547,7 +547,6 @@ def getSvcAcctCredentials(scopes, act_as):
     credentials = google.oauth2.service_account.Credentials.from_service_account_info(GM_Globals[GM_OAUTH2SERVICE_JSON_DATA])
     credentials = credentials.with_scopes(scopes)
     credentials = credentials.with_subject(act_as)
-    # TODO: figure out how to set user agent
     GM_Globals[GM_OAUTH2SERVICE_ACCOUNT_CLIENT_ID] = GM_Globals[GM_OAUTH2SERVICE_JSON_DATA]['client_id']
     return credentials
   except (ValueError, KeyError):

src/google_auth_httplib2.py

@@ -112,9 +112,13 @@ def __call__(self, url, method='GET', body=None, headers=None,
                 'Set the timeout when constructing the httplib2.Http instance.'
             )
 
+        if self.user_agent:
+          if headers.get('user-agent'):
+            headers['user-agent'] = '%s %s' % (self.user_agent, headers['user-agent'])
+          else:
+            headers['user-agent'] = self.user_agent
+
         try:
-            if self.user_agent is not None:
-                headers['user-agent'] = self.user_agent
             _LOGGER.debug('Making request: %s %s', method, url)
             response, data = self.http.request(
                 url, method=method, body=body, headers=headers, **kwargs)
@@ -172,7 +176,6 @@ def __init__(self, credentials, http=None, user_agent=None,
             http = _make_default_http()
 
         self.http = http
-        self.user_agent = user_agent
         self.credentials = credentials
         self.user_agent = user_agent
         self._refresh_status_codes = refresh_status_codes
@@ -191,8 +194,6 @@ def request(self, uri, method='GET', body=None, headers=None,
         # Make a copy of the headers. They will be modified by the credentials
         # and we want to pass the original headers if we recurse.
         request_headers = headers.copy() if headers is not None else {}
-        if self.user_agent is not None:
-            request_headers['user-agent'] = self.user_agent
 
         if self.user_agent:
           if request_headers.get('user-agent'):