Fix field names with special characters in where conditions

[andrykonchin]

Fix using fields with special characters in where conditions. It's a regression introduced in #655 (v3.10.0).

Examples:

User.where(_sortKey: 'a')
User.where('last:name': 'a')

Errors:

Invalid KeyConditionExpression: Syntax error; token: "_", near: "AND _sortKey"
Invalid FilterExpression: Syntax error; token: ":name", near: "last:name ="

The following characters are considered special in attribute names (according to the DynamoDB documentation) - :, #, ., -.

The dot (.) still is not supported by Dynamoid in where conditions.

Close #808