Add a simple benchmark.

Current figures (MBP i5, 16GB) are:
                                  user     system      total        real
With 1 array and 1 string values  1.020000   0.000000   1.020000 (  1.020917)
With 5 array and 5 string values  0.990000   0.010000   1.000000 (  0.992938)
With 10 array values              0.640000   0.000000   0.640000 (  0.648161)
With 10 string values             0.480000   0.000000   0.480000 (  0.479528)
Improve helper’s performance.

Resulting figures (MBP i5, 16GB):

                                  user     system      total        real
With 1 array and 1 string values  0.720000   0.000000   0.720000 (  0.723582)
With 5 array and 5 string values  0.720000   0.000000   0.720000 (  0.720296)
With 10 array values              0.450000   0.000000   0.450000 (  0.451907)
With 10 string values             0.360000   0.000000   0.360000 (  0.365057)
Add a test case for a scenario when multiple keys are used.
Replace #should with #is_expected for consistency.
Merge pull request #1 from temochka/performance-fix

Improved performance and unit tests coverage