Fix flaky test which assumes total_render_time can't be 0.

jruby has millisecond precision for Time.now, so total_render_time can be 0
due to this lack of precision.
Merge pull request #449 from Shopify/fix-flaky-total-render-time-test

Fix flaky test which assumes total_render_time can't be 0.
Remove Liquid::FilterNotFoundError since it is never raised.
Merge pull request #448 from Shopify/remove-unused-filter-not-found-error

Remove Liquid::FilterNotFoundError since it is never raised.
Check and handle when a tainted variable is used
Auto-untaint variables passed through 'escape'
Add tainting tests
Address code review comments

- clean up comment wording
- fix potentially leaky tests
Regression test for including assignments
Merge pull request #450 from Shopify/additional_test_for_includes

Regression test for including assignments
Merge pull request #440 from Shopify/drop-tainting

Variable tainting
Avoid modifying the default resources limits hash.
Merge pull request #453 from Shopify/no-modify-default-resource-limit

Avoid modifying the default resources limits hash.
Line numbers for all parse errors
Test line number of errors in nested blocks.
Merge pull request #455 from Shopify/parse_error_line_numbers

Line numbers for all parse errors
Remove unused Block#end_tag method.

Although the method is called, it is defined with an empty body and not
overridden to do anything else.