Merge and add tests related to 5215
`#as_json` isolates options when encoding a hash. Closes #8182

Setting options in a custom `#as_json` method had side effects.
Modifications of the `options` hash leaked outside and influenced
the conversion of other objects contained in the hash.
cleanup, removed whitespace from CHANGELOG
Debugger gem doesn't work with patchlevel < 327
`#pluck` can be used on a relation with `select` clause.

Closes #7551
Update production.rb documentation: only files in app/assets are in config.assets.precompile
Update production.rb: Split long comment in two lines
Merge pull request #8184 from vipulnsward/add_merge_tests

Merge and add tests related to #5215
depend on voloko/sdoc again

voloko/sdoc seems to be maintained again, in particular
the issues that made us fork it are resolved.
Updated the preformance_testing guide to reflect the fact that prof-rails no longer needs to be fetched from github
only clear caches if we are actually connected to the database
Merge pull request #8185 from senny/8182_as_json_options_stick_around

`#as_json` isolates options when encoding a hash. Closes #8182
Ruby 2.0.0 defaults source encoding to utf-8 so we need to specifically tag this file with us-ascii
Add a regression test on #8195
Merge pull request #8180 from freerange/mocha-fixes-supporting-only-0.13-onwards

Simplify code by taking advantage of latest mocha (v0.13.0).
Use new mocha version and add changelog entry for #8180
Merge pull request #8201 from nikitug/regression_test_on_ar_cache_key_precision

Add a regression test on #8195
Merge pull request #8176 from senny/7551_pluck_with_select

`#pluck` can be used on a relation with `select` clause (#7551)
Regression test for #7238
Merge pull request #8202 from nikitug/regression_test_on_dynamic_finder_result

Regression test for #7238