Fix SelfDrop equality

[karreiro]

Liquid::SelfDrop currently behaves like a wrapper object for equality, even though every meaningful operation delegates back to the captured context. That means two separate self lookups from the same context compare as different values, leaking implementation detail into both Ruby callers and template behavior:

ctx = Liquid::Context.new({})
a = ctx.find_variable("self")
b = ctx.find_variable("self")

[a == b, a.equal?(b), a == a]
# Before: [false, false, true]
# After:  [true,  false, true]

This change makes SelfDrop equality compare the captured context identity, and aligns eql?/hash with the same rule so Ruby hash-key semantics stay coherent.

[aswamy]

Looks good 👍 I also checked out the test on liquid-spec and seem consistent.