Constraint::check() uses wrong indefinite article in error message to express the type constraint

While making use of composer's validation method (which uses you library), it came to my attention that the error messages given are wrong for the indefinite article.
E.g. it says:

> NULL value found, but a integer is required

Which is wrong because the wrong indefinite article is used (compare: [When to Say "a" or "an"](http://www.englishclub.com/pronunciation/a-an.htm) etc.).

The correct form is:

> NULL value found, but **an** integer is required [highlight by me]