Not sure. OkHttp's HttpURLConnectionImpl does this, the RI does it. Figured I'd do it too. 

The Android implementation of >URLConnection< actually forbids all request property access if connected (and I assume the RI does the same), but HttpURLConnectionImpl overrides the behavior and relaxes it for getRequestProperty() but not getRequestProperties().

I assume the original intent was that that URLConnections may not hang onto the request properties so access to them could not be guaranteed. I suspect the RI inconsistency was an oversight and OkHttp behavior was inherited from harmony slavishly implementing the same behavior?

I can change it - let me know. Looks like OkHeaders.toMultimap() would be all I'd need + test updates.