avoiding number_format expects double error

Under some circumstances `number_format` can return an error since it expects the var type to be `double`.

Now we're casting to make sure we get the proper type of var.
Merge pull request #2306 from curtismchale/patch-1

avoiding number_format expects double error
Should solve #2310
Fix currency sign in price formatting bug
Merge pull request #2318 from ghost1227/issue/2316

Fix bug with currency sign in price
Add unit test for issue 2316
Add foreign currencies to unit tests
Add sanitization to variable prices
Add class name to "remove" button in cart template.

The remove button in the default cart template doesn't have a class name. This makes it awkward to add a css style to it while developing themes. I have added a class name of "edd-cart-remove-item-btn" so that styles may easily be applied.
Update German translation file with proper string. Fixes #2307
Merge pull request #2319 from ghost1227/issue/2316

Update unit tests, sanitize variable prices
Tweak formatting and check that price isn't empty when saving variable prices. #2316
Hyphens to Underscores on Class Name
Merge pull request #2321 from mintplugins/master

Add class name to "remove" button in cart template.
Merge branch 'master' into release/2.0
Pass proper variable to test_discount_is_valid_when_purchasing(). #2302
Better defensive coding in edd_get_discount_by(). #2302
Disable test for testing. #2302
Better defensive coding for edd_is_discount_used(). #2302
Check that the ID passed isn't empty before calling get_post() in edd_get_discount()