Add support for configuring the signature algorithm with HTTP post

[orf]

Support for customizing the signature algorithm with HTTP redirects was added here: 441b8d1

However there is no support for customising the algorithm with HTTP posts. The only workaround is to do this:

from saml2 import xmldsig
xmldsig.sig_default = xmldsig.SIG_RSA_SHA512

Is it possible to add support for customising the signature algorithm with HTTP post bindings as well? It seems quite simple, I can perhaps add support for it in a merge request if it is as simple as I think it is.

[peppelinux]

Discussion about this in pysaml2 repository is here:
IdentityPython/pysaml2#495

It could be a usefull to introduce it directly in pysaml2 configuration manager.

[peppelinux]

Following the pySAML2 PR and the latest contributions in what will have as the v0.18.2 release, we got this problem fixed, please let us know if you think that we could also improve the actual approach