Only use MethodLiteral in condition expressions

[dylanahsmith]

Problem

The MethodLiteral literal value was added in #592 which described it as a hacky solution:

Not sure if this is a good fix, feels kinda hacky, but seems to work.

One way in which it is hacky is that it is only relevant for boolean comparison equality expressions and isn't relevant for all expressions.

In liquid-c (Shopify/liquid-c#60), I would like Liquid::Expression.parse to return a Liquid::C::Expression object that can be rendered more efficiently and I would like to avoid coupling this work to these MethodLiteral objects.

Solution

Add a Liquid::Condition.parse_expression method that wraps Liquid::Expression.parse so that it will return a MethodLiteral object for empty or blank literals and will otherwise delegate to Liquid::Expression.parse. This way the values for these literals that are returned by Liquid::Expression.parse can just be the empty strings that MethodLiteral#to_liquid returns.

[tjoyal]

LGTM