Allow parsing multiple strings using same imports.

[vanderlee]

If you use a single Less instance to parse multiple strings and these strings share @import statements, only the first parse will actually execute the @import.

[glensc]

added in https://github.com/MarcusSchwarz/lesserphp/releases/tag/v0.5.1

[Krinkle]

Thanks. This looks good, but I want to make sure we confirm this with a unit test.