Fix bug in adding Authorization header for client authentication in a certain case.

There is a bug in adding an Authorization header in case of using client_secret_basic.
To fix it, a return value of ClientSecretBasic::construct is changed to the same as the return value of BearerHeader::construct.
Support configurable token_endpoint_auth_method for rp3.

Currently, rp3 supports only 'client_secret_post' for token_endpoint_auth_method.
To support 'client_secret_basic', token_endpoint_auth_method attribute is changed to be configurable from conf.py.
Update test to conform the change of ClientSecretBasic::construct.
Merge remote-tracking branch 'upstream/master' into topic-oidc-example-rp-basic-auth
Merge pull request #18 from dajiaji/topic-oidc-example-rp-basic-auth

Update rp3 to support client_secret_basic.