make schema attr lookup case insensitive

If the ODBC connection string doesn't specifically
set the `schema` connection param name as all lowercase,
the lookup for `current_schema` will result in `nil` which
in turn leads to various errors that depend on this value
being properly set.

This lookup was only checking for strictly `schema` attribute,
however, the ODBC connection string seems to allow for some
flexibility in how its param names are formatted - we've seen
these attributes writen as "schema", "Schema", or "SCHEMA"
(e.g. this repo's README and code examples show the
uppercase variant for its ODBC strings [1][2]).

This makes the lookup case-insensitive to allow
all variants of formatting for this attribute name.

[1] https://github.com/doximity/odbc_adapter/blob/d36c64ee2973942558b960c706848db06f50ffde/README.md#L45
[2] https://github.com/doximity/odbc_adapter/blob/a18a9ef02d27488a3b2b31ad7c695743bc2ff098/lib/active_record/connection_adapters/odbc_adapter.rb#L56

lib/odbc_adapter/schema_statements.rb

@@ -193,7 +193,7 @@ def current_database
     end
 
     def current_schema
-      @config[:driver].attrs['schema']
+      @config[:driver].attrs.transform_keys(&:downcase)['schema']
     end
 
     def name_regex(name)