Fix for parsing of conditions in assign statements

[mhalada-from-meiro]

Fix for parsing of condition in Liquid assign statements. Fixes https://github.com/osteele/liquid/issues/76

References:

• https://github.com/Shopify/liquid/issues/1102

Checklist

• [x] I have read the contribution guidelines.
• [x] make test passes.
• [x] make lint passes.
• [x] New and changed code is covered by tests.
• [x] Performance improvements include benchmarks.
• [ ] Changes match the documented (not just the implemented) behavior of Shopify.