Object allocation during parsing method name

Hey everyone!

One day we decided to profile our rails application where we heavily rely on ElasticSearch and Hashie.

I had a look into "Allocated String Report" and noticed that we have allocated the same string `(.*?)([?!=_]?)$`  *50 000 times*, and then I noticed that it's inside `Hashie::Mash`.

This pull-request reduces amount of allocated objects on calls with suffixes.

Benchmark code:

```ruby
mash = Hashie::Mash.new
mash.let = 'store'

N = 1000000

before = GC.stat(:total_allocated_object)
N.times { mash.let? }
after  = GC.stat(:total_allocated_object)

puts (after - before) / N
puts GC.stat
```

Before change:
```
18
{:count=>670, :heap_used=>145, :heap_length=>145, :heap_increment=>0, :heap_live_slot=>58961, :heap_free_slot=>138, :heap_final_slot=>0, :heap_swept_slot=>21984, :heap_eden_page_length=>145, :heap_tomb_page_length=>0, :total_allocated_object=>18219623, :total_freed_object=>18160662, :malloc_increase=>18656, :malloc_limit=>16777216, :minor_gc_count=>667, :major_gc_count=>3, :remembered_shady_object=>426, :remembered_shady_object_limit=>574, :old_object=>31820, :old_object_limit=>42556, :oldmalloc_increase=>2044464, :oldmalloc_limit=>16777216}
```

After change:
```
9
{:count=>319, :heap_used=>145, :heap_length=>145, :heap_increment=>0, :heap_live_slot=>59035, :heap_free_slot=>68, :heap_final_slot=>0, :heap_swept_slot=>2923, :heap_eden_page_length=>145, :heap_tomb_page_length=>0, :total_allocated_object=>9219622, :total_freed_object=>9160587, :malloc_increase=>10544, :malloc_limit=>16777216, :minor_gc_count=>316, :major_gc_count=>3, :remembered_shady_object=>424, :remembered_shady_object_limit=>574, :old_object=>27789, :old_object_limit=>42554, :oldmalloc_increase=>1019152, :oldmalloc_limit=>16777216}
```

Twice less allocated objects for checking the existence, also no more recreation of constant string after every method access.
 
Would be happy to hear any feedback!