Some small fixes for MySQL and for upcasing identifiers on JDBC

This changes Dataset#upcase_identifiers? to lazily load the setting.
Previously, it was handled in Database#initialize, but on JDBC (and
other adapters that have subadapters), the method that provides the
default hasn't been included in the class when Database#initialize
tried to use it, so this would end up with identifiers being upcased
when using MySQL, PostgreSQL, or SQLite on JDBC.

This fixes Database#alter_table for MySQL, for the drop_index case,
and fixes the MySQL schema parser to handle the fact that the table
name provided to schema_parse_table is now a string instead of a
symbol.

lib/sequel_core/adapters/shared/mysql.rb

@@ -20,7 +20,7 @@ def alter_table_sql(table, op)
         when :set_column_type
           "ALTER TABLE #{quote_schema_table(table)} CHANGE COLUMN #{quote_identifier(op[:name])} #{quote_identifier(op[:name])} #{type_literal(op)}"
         when :drop_index
-          "#{drop_index_sql(table, op)} ON #{quoted_table}"
+          "#{drop_index_sql(table, op)} ON #{quote_schema_table(table)}"
         else
           super(table, op)
         end
@@ -80,7 +80,7 @@ def upcase_identifiers_default
 
       # Use the MySQL specific DESCRIBE syntax to get a table description.
       def schema_parse_table(table_name, opts)
-        self["DESCRIBE ?", table_name].map do |row|
+        self["DESCRIBE ?", SQL::Identifier.new(table_name)].map do |row|
           row.delete(:Extra)
           row[:allow_null] = row.delete(:Null) == 'YES'
           row[:default] = row.delete(:Default)

lib/sequel_core/database.rb

@@ -58,7 +58,6 @@ def initialize(opts = {}, &block)
       @opts = opts
 
       @quote_identifiers = opts.include?(:quote_identifiers) ? opts[:quote_identifiers] : @@quote_identifiers
-      @upcase_identifiers = opts.include?(:upcase_identifiers) ? opts[:upcase_identifiers] : (@@upcase_identifiers.nil? ? upcase_identifiers_default : @@upcase_identifiers)
       @single_threaded = opts.include?(:single_threaded) ? opts[:single_threaded] : @@single_threaded
       @schemas = nil
       @prepared_statements = {}
@@ -475,7 +474,7 @@ def typecast_value(column_type, value)
 
     # Returns true if the database upcases identifiers.
     def upcase_identifiers?
-      @upcase_identifiers
+      @upcase_identifiers ||= @opts.include?(:upcase_identifiers) ? @opts[:upcase_identifiers] : (@@upcase_identifiers.nil? ? upcase_identifiers_default : @@upcase_identifiers)
     end
 
     # Returns the URI identifying the database.