Bugfix/fix compilation error on ios for 8.0.0.beta9

[nobodyabcd]

Proposed changes

@cybergrind, please have a look at this PR. this is a fix to issue Issue 585

Currently, user on Xcode could not build tipsi-stripe version 8.0.0.beta9 because a typo here:

case STPPaymentMethodTypeFPX: return @"fpx";

fpx is not currently supported, it should be marked as type unknown rather than fpx.

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• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)

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[cybergrind]

hey, @fbartho ! Could you please look into recent PR's from @guanhuay and if they are ok, probably we need to release new beta