Fixed interpolate() to pass tests

The current implementation of the `interpolate()` provided in the PSR-3 RFC breaks tests of https://github.com/php-fig/log/blob/master/Psr/Log/Test/LoggerInterfaceTest.php and the RFC itself:
> Every method accepts an array as context data. This is meant to hold any extraneous information that does not fit well in a string. The array can contain anything. Implementors MUST ensure they treat context data with as much lenience as possible. A given value in the context MUST NOT throw an exception nor raise any php error, warning or notice.

This PR fix that.