75% fewer Symbol#to_s calls in merge_attributes

In awsm, create_current_user ends up calling Symbol#to_s 1.6 million times
per invocation. 1.4 million of those come from Cistern's merge_attributes.

This patch converts the incoming key to a string and a symbol variant, once
each, and only uses those for its work. On average each key version of the
symbol was being converted to a string again 4 times in this method.

---

Awsm travis runs with and without:

  with optimization: ran for 14 hrs 48 min 31 sec
  https://magnum.travis-ci.com/engineyard/awsm/builds/7465897

  without optimization: ran for 17 hrs 9 min 48 sec
  https://magnum.travis-ci.com/engineyard/awsm/builds/7463787

So, ~86% of the time to run whole awsm suite with this change.

---

Before: Symbol#to_s called 1.6 million times in each create_current_user

Calls:

Total: 18.778550

 %self      total      self      wait     child     calls  name
  9.01      3.179     1.692     0.000     1.487      775   Array#sort!
  6.32      1.187     1.187     0.000     0.000  1564786   Symbol#to_s
  3.56      0.669     0.669     0.000     0.000      944   <Module::Kernel>#caller
  3.07      0.577     0.577     0.000     0.000   565138   Kernel#hash

Time-per create_current_user benchmark

user       system     total       real
3.300000   0.060000   3.360000 (  3.495115)
2.980000   0.050000   3.030000 (  3.168694)
3.000000   0.040000   3.040000 (  3.189597)
3.020000   0.050000   3.070000 (  3.219720)
3.000000   0.050000   3.050000 (  3.191496)
3.010000   0.050000   3.060000 (  3.190507)
3.020000   0.050000   3.070000 (  3.206198)
2.990000   0.050000   3.040000 (  3.179688)
3.040000   0.050000   3.090000 (  3.231775)

---

After: Symbol#to_s called 0.4 million times in each create_current_user

Calls:

Total: 11.640456

 %self      total      self      wait     child     calls  name
  4.24      0.493     0.493     0.000     0.000   455451   Kernel#hash
  3.12      0.680     0.363     0.000     0.316      168   Array#sort!
  2.35      0.997     0.274     0.000     0.723    46069   DataMapper::Property#hash
  2.28      0.266     0.266     0.000     0.000   369464   Symbol#to_s

Time-per create_current_user benchmark

user       system     total       real
2.360000   0.060000   2.420000 (  2.562157)
1.970000   0.040000   2.010000 (  2.145008)
1.920000   0.050000   1.970000 (  2.091060)
1.870000   0.040000   1.910000 (  2.040251)
1.900000   0.050000   1.950000 (  2.070622)
1.820000   0.030000   1.850000 (  1.970871)
1.860000   0.040000   1.900000 (  2.011723)
1.850000   0.040000   1.890000 (  2.027080)
2.060000   0.040000   2.100000 (  2.277219)