Liquid::Drop should not return a string representation of standard ruby objects

#### Problem

When parsing an instance of a liquid drop (ex: `{{ product }}`), the result is a standard ruby object `"#<ProductDrop:0x007fcdec010e98>"`. This feels wrong for a template engine. This is an implementation detail of Ruby. The end-user or template builder should see something nicer.

#### Solution

Provide a very simple default for liquid drops `to_s`. Which can be easily overridden in individual implementations. This means parsing an instance of a liquid drop will return `"ProductDrop"` instead of a standard ruby object.

Review/Feedback: @fw42 @camilo 
CC: @jamesmacaulay