Before this change 0.00 and '0.00' are treated differently. Floats from the database are returned as the string
version while doing calculations will normally end up as floats.

This causes output differences on pages like order totals or invoices where there is a mix of calculated values
and database values.

	Number::currency(0.00, 'GBP') -> &#163;0.00
	Number::currency('0.00', 'GBP') -> 0p

Both versions will return `&#163;0.00` (or whatever 0 is configured to return).
Merge pull request #1071 from dogmatic69/currency-format

'0.0' and 0 should be formatted consistent.

Fixes #3525