Bump goxmldsig to 1.2.0

[lafriks]

saml/go.mod

Line 13 in 7444b81

github.com/russellhaering/goxmldsig v1.1.1

Otherwise there is currently build error because #428 was merged but dependency was not updated.

[crewjam]

Bumping to version 1.2.0 pulls in this commit which changes how comments are handled.

This causes the TestSPRejectsInjectedComment test to fail with both comments enabled and disabled. That test was added as a regression to a security issue. I don't think we can bump to v1.2.0 until we understand why that test fails.