Prettify column defaults

[drwl]

Fixes #45.

When I extracted out column annotation building [1], I changed the way the default value was read. Model#column_defaults has the column default values that are casted appropriately, where as Column#default is uncasted and straight from the database, most of the time as a String. It's important to note that the various adapters (sqlite, mysql, postgres) implement the columns differently [2], especially as it relates to Column#default and Model#column_default values.

This PR restores parity with the original gem on how defaults are handled. It also adds tests for the various adapters, because the implementations vary under the hood.

As an example, the mysql adapter sets the value for an integer field without a default to be 0 in Model#column_defaults, where as sqlite and postgres set it to be nil.

[1] https://github.com/drwl/annotaterb/pull/28/files#diff-92c15977800c36cb911c1d7971ede0da49dae93b0c9e555777a5ca67ee81d93dR12-R14

[2] https://gist.github.com/drwl/ef274a763b773d132abfdb95d32347d2

[drwl]

Some notes from debugging this. The default value as a String gets passed from ActiveRecord internally itself.

# The column object we get from AR has a default value that's String
# [4] pry(#<AnnotateRb::ModelAnnotator::ColumnAnnotation::AttributesBuilder>)> rcol = @column.instance_variable_get("@column")
# => #<ActiveRecord::ConnectionAdapters::PostgreSQL::Column:0x000000010e19ea60
#  @collation=nil,
#  @comment=nil,
#  @default="0",
#  @default_function=nil,
#  @generated="",
#  @name="duration_minutes",
#  @null=true,
#  @serial=nil,
#  @sql_type_metadata=
#   #<ActiveRecord::ConnectionAdapters::SqlTypeMetadata:0x000000010e19f140
#    @limit=4,
#    @precision=nil,
#    @scale=nil,
#    @sql_type="integer",
#    @type=:integer>>

# Internally, it also is a String
# [4] pry(#<ActiveRecord::ConnectionAdapters::PostgreSQLAdapter>)> field
# => ["duration_minutes", "integer", "0", false, 23, -1, nil, nil, ""]

[drwl]

As a side note, I found out that column names should actually be strings. There are some tests that use mock_column(:some_symbol, ...) and this leads to incorrect test setup + expected output.

Will handle in another PR.