fix(115): FrozenError on BuildableSearch

[jessedoyle]

Hello!

I noticed that building queries dynamically via #build_query will mutate the underlying query string passed in to the key_expr method when Aws::Record is creating the prepared statement.

When using the frozen_string_literal: true directive, this causes the following exception to be raised:

FrozenError (can't modify frozen String: ...)

This PR resolves the issue by modifying the private methods that create a prepared statement to return a new string instance rather than mutate the underlying string.

Issue #, if available:

resolves #115.

Description of changes:

• Write a failing test for the issue described in #115.
• Add vendor/bundle and .bundle to the gitignore file for those of us that vendorize their dependencies.
• Fix up some whitespace issues in CONTRIBUTORS.md
• Modify the private methods of _apply_values and _key_pass to return a new string instance of a prepared statement after each pass - this way we can avoid calling mutative string methods (i.e. String#gsub!)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[alextwoods]

Thanks for submitting this! Overall the change looks reasonable, but it looks like our travis based CI test suite has stopped working. We're working on fixing that and should be able to review this week.

[mullermp]

I've migrated to GH actions. Can we force a rebuild somehow?

[jessedoyle]

@mullermp - I’ll rebase the upstream changes into the branch. That should trigger a rebuild!

[jessedoyle]

@mullermp - Rebased! Looks like GitHub Actions just needs approval to run.

[mullermp]

Thanks for your contribution. We will release this sometime this week.