Duplicate code for ECP request

[erakli]

There's same code pieces (with minor differences):

https://github.com/erakli/pysaml2/blob/master/src/saml2/client_base.py#L783 https://github.com/erakli/pysaml2/blob/master/src/saml2/ecp.py#L43

I think, we need to eliminate duplication and merge all ECP stuff in one place.

[peppelinux]

I think you're right and probably the ECP support is taking a back seat in this project. Please do a PR for this if you can, AFAIK it seems to be trivial starting from what you show us