Fixed Mysql2::Client#query return value to be uniquely determined (#564)

The return value of `Mysql2::Client#query` is determined by the combination of
the `as` argument of the `self.new` method and the `query` method.
Fixed it so that it can be expressed using Generics.

gems/mysql2/0.5/_test/query.rb

@@ -0,0 +1,46 @@
+require "mysql2"
+
+as = :array
+Mysql2::Client.new(as: as).query("SELECT * FROM users").each do |row|
+  if row.is_a?(Array)
+    row[0] # Array
+  elsif row.is_a?(Hash)
+    row[:id] # Hash
+  end
+end
+
+Mysql2::Client.new.query("SELECT * FROM users").each do |row|
+  row[:id] # Hash
+end
+
+Mysql2::Client.new.query("SELECT * FROM users", as: :hash).each do |row|
+  row[:id] # Hash
+end
+
+Mysql2::Client.new.query("SELECT * FROM users", as: :array).each do |row|
+  row[0] # Array
+end
+
+Mysql2::Client.new(as: :hash).query("SELECT * FROM users").each do |row|
+  row[:id] # Hash
+end
+
+Mysql2::Client.new(as: :hash).query("SELECT * FROM users", as: :hash).each do |row|
+  row[:id] # Hash
+end
+
+Mysql2::Client.new(as: :hash).query("SELECT * FROM users", as: :array).each do |row|
+  row[0] # Array
+end
+
+Mysql2::Client.new(as: :array).query("SELECT * FROM users").each do |row|
+  row[0] # Array
+end
+
+Mysql2::Client.new(as: :array).query("SELECT * FROM users", as: :hash).each do |row|
+  row[:id] # Hash
+end
+
+Mysql2::Client.new(as: :array).query("SELECT * FROM users", as: :array).each do |row|
+  row[0] # Array
+end

gems/mysql2/0.5/client.rbs

@@ -1,5 +1,5 @@
 module Mysql2
-  class Client
+  class Client[ResultType]
     self.@default_query_options: untyped
 
     @read_timeout: untyped
@@ -14,6 +14,11 @@ module Mysql2
 
     def initialize: (Hash[(String | Symbol), untyped] opts) -> void
 
+    def self.new: (as: :hash, **untyped) -> Mysql2::Client[Mysql2::ResultAsHash]
+                | (as: :array, **untyped) -> Mysql2::Client[Mysql2::ResultAsArray]
+                | (as: Symbol, **untyped) -> Mysql2::Client[Mysql2::ResultAsHash | Mysql2::ResultAsArray]
+                | (**untyped) -> Mysql2::Client[Mysql2::ResultAsHash]
+
     def parse_ssl_mode: (untyped mode) -> untyped
 
     def parse_flags_array: (untyped flags, ?::Integer initial) -> untyped
@@ -27,7 +32,9 @@ module Mysql2
     # and performance_schema.session_account_connect_attrs
     def parse_connect_attrs: (untyped conn_attrs) -> (::Hash[untyped, untyped] | untyped)
 
-    def query: (String sql, ?::Hash[untyped, untyped] options) -> Mysql2::result
+    def query: (String sql, as: :hash, **untyped) -> ResultAsHash
+             | (String sql, as: :array, **untyped) -> ResultAsArray
+             | (String sql, **untyped) -> ResultType
 
     def query_info: () -> (::Hash[untyped, untyped] | untyped)
 

gems/mysql2/0.5/result.rbs

@@ -1,6 +1,5 @@
 module Mysql2
   type row_value_type = String | Integer | nil
-  type result = ResultAsHash | ResultAsArray
 
   class ResultAsHash
     attr_reader server_flags: untyped