Fix article for generator name

Use 'some' instead of 'a/an'.
Add some tests to enumerate how extensions should be stored in the schema output
improve tests to check for existence of extensions method, and skip testing dumped extensions if they are unsupported by the database
add ActiveRecord::AbstractAdapter#extensions and ActiveRecord::ConnectionAdapters::PostgreSQLAdapter#extensions to allow dumping of enabled extensions to schema.rb, add ActiveRecord::SchemaDumper#extensions to dump extensions to schema.rb
Merge pull request #9203 from jaggederest/dumping_pgsql_extensions

Adding database extension support to schema.rb
Add changelog entry for #9203 about schema dumper with db extensions

[ci skip]
Call super to use the abstract adapter implementation instead
Fix indentation of extensions in schema
Add blank line after extensions to separate from tables in schema
Do not print anything related to extensions when they don't none exist

When extensions are supported but there's no one enabled in the
database, we should not print anything related to them in schema.rb.
Run schema dumper extensions without creating real extensions

There is no need to create the extension in the database just to test if
it's dumped, we can stub that instead.
active_record: Quote numeric values compared to string columns.
Skip schema dumper extensions test if connection does not support it

Travis failed because the postgresql version installed there does not
support extensions, so we just need to skip this for now.
Merge pull request #9207 from dylanahsmith/mysql-quote-numeric

active_record: Quote numeric values compared to string columns.
Merge pull request #9185 from tricknotes/fix-article-for-generator-name

Fix article for generator name
Change behaviour with empty hash in where clause
Merge pull request #9220 from robertomiranda/where-with-empty-hash

Active Record: Change behaviour with empty hash in where clause
Change behaviour with empty array in where clause
Merge pull request #9216 from robertomiranda/where-with-empty-array

Activer Record:  Change behaviour with empty array in where clause
Improve changelog entries in AR

Describe better the changes related to raising errors with empty
array/hash values in where clauses.

[ci skip]