Changed string return type to float according to JWT specs

[enrico-atzeni]

NumericDate of exp and iat claims do not respect JWT specifications as of here
exp: https://tools.ietf.org/html/draft-ietf-oauth-json-web-token-32#section-4.1.4
iat: https://tools.ietf.org/html/draft-ietf-oauth-json-web-token-32#section-4.1.6

both says

[...] Its value MUST be a number containing a NumericDate value. [...]

but MicrosecondBasedDateConversion's convertDate method returns a string instead of a number.

return $seconds . '.' . $microseconds;

Fixed it casting the string to a float:

return (float) ($seconds . '.' . $microseconds);

[Slamdunk]

Duplicate of #618
See also #617 #623

[enrico-atzeni]

thanks @Slamdunk, didn't find it before, I will use the fix mentioned in #618

[lcobucci]

Hello folks, thanks to @yassinrais we managed to fix this compatibility issue whilst still preserving the microseconds precision. We already released new versions to solve the problem.