Fix for getForeignKeys()/getAllForeignKeys() methods

[lvivier]

Hello! This PR changes "METADATA-FOREIGNKEYS" to "METADATA-FOREIGN_KEYS" in metadata.coffee. Per RCP 87 - RETS 1.7.2 Errata Document, the latter is the correct value:

Erratum 8

The Foreign Key Metadata element has an inconsistency in the 1.7.2. The tag label for the metadata content is referenced in three places.
METADATA-FOREIGN_KEYS is specified on page 11-9 of Section 11.2.3
METADATA-FOREIGN_KEYS is specified in the RETS 1.7.2 DTD on page 6 in line 273
METADATA-FOREIGNKEYS is specified on page B-2 of Appendix B-2
It is also referenced in Table 11-12, Metadata Content, Field ForeignKeyName in the Description column as METADATA-FOREIGNKEYS.
The correct use is METADATA-FOREIGN_KEYS.

RCP 87 can be found in the RETS 1.8 Specification, Appendix E.

RETS servers I'm dealing with do respond with the foreign keys metadata table after this change.