Fix building a list of upgrade operations for model classes by using case insensitive field type names

[vladbelousoff]

Before the change, the library generated migration files using field_type from peewee, for example, UUIDField and IntegerField have uppercased field_type values:

class UUIDField(Field):
    field_type = 'UUID'
    ...

class IntegerField(Field):
    field_type = 'INT'
    ...

Using this field will generate the following migration file:

def upgrade(migrator):
    with migrator.create_table('performance_report_stat_unit') as table:
        table.foreign_key('UUID', 'session_uid', on_delete=None, on_update=None, references='performance_report_session.uid')

However, when applying migration, the type is searched using the values from FIELD_TO_PEEWEE that only contains lowercase values:

PEEWEE_TO_FIELD = {
    peewee.BareField: 'bare',
    peewee.BigIntegerField: 'biginteger',
    peewee.BlobField: 'blob',
    peewee.BooleanField: 'bool',
    peewee.CharField: 'char',
    peewee.DateField: 'date',
    peewee.DateTimeField: 'datetime',
    peewee.DecimalField: 'decimal',
    peewee.DoubleField: 'double',
    peewee.FixedCharField: 'fixed',
    peewee.FloatField: 'float',
    peewee.ForeignKeyField: 'foreign_key',
    peewee.IntegerField: 'int',
    peewee.AutoField: 'primary_key',
    peewee.SmallIntegerField: 'smallint',
    peewee.TextField: 'text',
    peewee.TimeField: 'time',
    peewee.TimestampField: 'int',
    peewee.UUIDField: 'uuid',
}

# Only available on peewee >= 3.3.4
if hasattr(peewee, 'BinaryUUIDField'):
    PEEWEE_TO_FIELD[peewee.BinaryUUIDField] = 'bin_uuid'

FIELD_TO_PEEWEE = {value: key for key, value in PEEWEE_TO_FIELD.items()}

Thus, we try to get a value using an incorrect key.
I suggest using casting to lowercase when searching and generating values.