Allow README.TXT files in metadata directory

[JornWildt]

Should anyone, by accident or on purpose, put a non-metadata file in the metadata file directory, the OIOSAML simply breaks down.

We like to add a README.TXT in the directory to explain it's purpose, and for this we modified SAML20FederationConfig ever so slightly with an added file mask of "*.config":

internal void Initialize()
  string[] files = Directory.GetFiles(MetadataLocation, "*.config");

In addition to this, it would be nice if OIOSAML handled non-metadat files by signalling a clear error message, indicating what is wrong.

[thomasnymand]

Addressed in #82.

The root cause was that ParseFile returns null for a file that isn't SAML 2.0 metadata, but HandleCreateIdp/HandleUpdateIdp dereferenced the result immediately — so a single non-metadata file (e.g. a README) in the metadata directory threw a NullReferenceException that aborted configuration loading.

These call sites now skip files that don't parse as metadata instead of failing, so a README.TXT (or any other file) can be kept in the directory. ParseFile already emits a trace entry ("Probably not a SAML2.0 metadata file") identifying which file was rejected and why, so the clear error message you asked for is provided.

Rather than filtering on a fixed extension like *.config, code skips anything that isn't valid metadata, so fix is in place regardless of the file extension used for the actual metadata files. As a side effect this also resolves the intermittent NullReferenceException reported in #72.

Thanks for the report!