FIX preserve enum values with correct escaping

[NightJar]

Fixes #45 Allows SQLite to support Enum values with the backslash character (\) in them as a default - e.g. DBClassName used for Polymorphic relationships.

Enum values are themselves enumerated in sqlite as they are not supported as a type. This leads to values being stored in their own table, and a regular TEXT field being used in a MySQL ENUM's stead. The default value for this field was being escaped with custom string replacement, and erroneously relacing the backslash (a redundant operation). This lead to invalid Fully Qualified Class Names in SilverStripe 4, which is a required trait for polymorphic relationships. As a result any polymorphic relationship not set on first write would then proceed to cause an execution error the next time the dataobject with the relationship was fetched from the database. By using the PHP supplied escape function for SQLite3 we can avoid this, and restore functionality.

Relevant section of SQLite documentation to justify the removal of escaping various characters, such as the backslash:

A string constant is formed by enclosing the string in single quotes ('). A single quote within the string can be encoded by putting two single quotes in a row - as in Pascal. C-style escapes using the backslash character are not supported because they are not standard SQL.

https://www.sqlite.org/lang_expr.html

[maxime-rainville]

Merging now, the build failures are related to a different issue.

[tractorcow]

Good job.