Add option to raise response validation exceptions

[quantus]

The current OneLogin_Saml2_Response.is_valid always collects the first raised exceptions and creates the error message from the exception content. I am using Raven (+ Sentry) to track all unhandled exceptions, so I would rather have the function just raise the first found validation error. When the exception is raised from the problem source Raven is able to capture local variables from the exception context. In my use case IdP:s can be dynamically added/removed/modified during the program runtime, so getting all the exception information is important to me.

This PR adds an optional raises argument to OneLogin_Saml2_Response.is_valid. When the argument is truthy, first found exception gets raised. As the option is optional this change is completely backwards compatible.

As I wrote test for this feature I noticed some oddness in some test code:

• Many tests contained code to test for raised exceptions event thou the functions didn't raise any exceptions. Fixed in cd2a6b4.
• Some tests used self.assertFalse(True) method to make sure that the exception was thrown. I refactored these tests in a9997f7.
• Some tests didn't make sure that the tested function actually raised the expected error. Fixed in c007ca7.

[coveralls]

Coverage increased (+0.005%) to 95.265% when pulling b029a8a on quantus:feature/re-raising-response-validation-exceptions into 054f8e6 on onelogin:master.

[pitbulk]

We should add this functionality to rest of is_valid methods (logoutrequest and logoutresponse)

[quantus]

That sounds like a good idea. I'll let you know when I have made the changes.

[pitbulk]

Now that we are able to raise exceptions, why no use them calling
valid = response.is_valid(self.get_request_data(), None, True)

[quantus]

When I was refactoring these tests I didn't take into consideration the changes that would come from adding the new argument. I wanted to keep the cleaning changes separate from the new feature to keep the commits simple.

I don't have any strong opinion about whether we should run these tests with the raising version. The default code calls these functions without the raises being set, but with the raises set we can more precisely make sure that the function failed with the wanted error.

I can change these functions to use the exception raising if you prefer that.

[coveralls]

Coverage increased (+0.01%) to 95.275% when pulling 16e38be on quantus:feature/re-raising-response-validation-exceptions into 054f8e6 on onelogin:master.

[quantus]

@pitbulk, I have now added the argument to other places and also made the current tests to test the raised exception. There were couple of response tests where the old code incorrectly tried to test that exception with wrong content was being thrown. The tests where I had to change the expected error message were:

• testIsInValidIssuer
• testIsInValidCert
• testValidateVersion

I also didn't touch the response test testIsInValidReference, as that test has some other issues. I wrote more about this issue to the issue #38.

[pitbulk]

Ok thanks for contribute, I will need some time to review

[pitbulk]

I will merge this on #42

[pitbulk]

@quantus can you review #42?