Do not exit from loadClass() early

In loadClass(), if the namespace/prefix matches, but the classfile is not
found, we were returning a boolean false immediately. However, if a more
specific namespace/prefix matches, but is later in the stack, we're
essentially ignoring it, making the class unreachable.

This patch removes the `return false` line in the `foreach` loop, allowing
a namespace registered later to match and potentially fulfill the autoloading.

There _will_ be a performance impact, which means it's always better to register
the more specific namespace earlier.
[#5783] Assertion on return value

- per @Ocramius
Merge PR #5783 to develop
Merge branch 'hotfix/standard-autoloader-stack' of git://github.com/weierophinney/zf2 into weierophinney-hotfix/standard-autoloader-stack

* 'hotfix/standard-autoloader-stack' of git://github.com/weierophinney/zf2:
  [#5783] Assertion on return value
  Do not exit from loadClass() early
Merge PR #5783 to develop
Merge branch 'weierophinney-hotfix/standard-autoloader-stack' into develop

* weierophinney-hotfix/standard-autoloader-stack:
  [#5783] Assertion on return value
  Do not exit from loadClass() early