Added failed test

Inserting Perl works well like:

  %div
    = undef || 0

But inlining it doesn't work:

  %div= undef || 0

I added two test and the first one is passed and
second one is failed now.
Return empty string only if exp is undefined
Add case about return value of Perl modifier exp
Do the same thing when there is no HTML tag
Remove 'use of uninitialized value...' warning
Merge pull request #31 from keedi/feature/separate-undef-and-false

Separate undef and false for Perl expression in Haml