Use correct case for emandate property in Gateway Account

[malles]

While setting up a transactions gateway data I got confused by the addEmanDate() method name and added the date of the mandate to that field.
However that field refers to the property emandate as per the docs

emandate
string
For SEPA Direct Debit direct orders (required), the unique emandate identifier (for your own administration).

I have edited the class to reflect the correct property name.
Due to the private visibility of the property, and PHP's case-insensitive function names this should not cause any problems in backward compatibility.

[vinodsowdagar]

@malles ,

Indeed right now it looks like the Emandate is a 'date' but it's not. Thank you for your PR! We're going to add it in the next release.

[danielcivit]

Thanks for your contribution @malles

• We moved your changes to a private - internal - mirror repository, and now they are merged and have been released publicly.
• We maintain recognition of your contribution through GitHub and within our CHANGELOG.
  • https://github.com/MultiSafepay/php-sdk/releases/tag/5.15.0
  • https://github.com/MultiSafepay/php-sdk/blob/master/CHANGELOG.md

Closing this PR, since the changes are included.