Updated style of Hash#get spec
Updated style of Hash#has_key? spec, added a couple tests
Updated style of Hash#has_value? spec, added a test
Make Hash#sample spec work more reliably
Added comment on requirements for classes including Hamster::Enumerable
Hash#merge can take a block to combine values
Fix Hash#flatten spec which can break depending on Hash iteration order
Fix Hash#hash spec which was based on faulty premise

One of the Hash#hash specs was based on the idea that :issued_at and :expires_at
would hash to the same bucket. That may have been true in an old version of Ruby,
but in up-to-date versions, the hash codes used by Symbols and other core classes
are different on each run.
Updated style of Hash#inspect spec
Updated style of Hash#invert spec, added a test
Updated style of Hash#key spec, added a test
Updated style of Hash#keys spec
Updated style of Hash#map spec, added a test
Updated style of Hash#merge spec
Updated style of Hash#min, #max, #min_by, #max_by spec. Added tests
Updated style of Hash.new spec, added a test
Updated style of Hash#none? spec, added a test
Updated style of Hash#partition spec
Updated style of Hash#put spec, added a test
Updated style of Hash#reduce spec