BridgeDB's txrecaptcha returns the "No bridges available!" page if 'captcha_response_field' is blank

I was writing unittests for the bridgedb.HTTPServer module and discovered this.

What is happening is:

• bridgedb.HTTPServer.ReCaptchaProtectedResource.render_POST(request) is called with blank strings for the 'captcha_challenge_field' and 'captcha_response_field' POST arguments.
• bridgedb.HTTPServer.CaptchaProtectedResource.render_POST(request) is called.
• bridgedb.HTTPServer.ReCaptchaProtectedResource.checkSolution(request) is called.
• bridgedb.HTTPServer.CaptchaProtectedResource.extractClientSolution(request) is called, and it returns a tuple of (_, _), which in Python has a boolean value of True.
• The empty strings return a bridgedb.txrecaptcha.RecaptchaResponse from bridgedb.HTTPServer.ReCaptchaProtectedResource.checkSolution() without hitting the callback function checkResponse().
• The RecaptchaResponse also evaluates to True, meaning that checkSolution(request) in render_POST() passes, and the server tries to render the RecaptchaResponse object as the list of bridges to give to the client, resulting in the "No bridges available!" webpage.

That sounds confusing. But I have a unittest to prove it happens, and the solution is really simple:

In bridgedb.CaptchaProtectedResource.render_POST():

-        if self.checkSolution(request):
+        if self.checkSolution(request) is True: