fix: add charset to table definitions (hschne#150)

For any databases that do not have utf8 as the default. Without this
setting the tables are created with the default, but are treated as
utf8 which can result in errors:

   ERROR 1366 (HY000): Incorrect string value: '\xE2\x84\xAF\x0A -...' for column 'response_body' at row 1

db/migrate/20210621185018_create_rmp.rb

@@ -2,7 +2,7 @@
 
 class CreateRmp < ActiveRecord::Migration[6.0]
   def change
-    create_table :rmp_profiled_requests do |t|
+    create_table :rmp_profiled_requests, charset: 'utf8' do |t|
       t.string :user_id
       t.bigint :start
       t.bigint :finish
@@ -23,7 +23,7 @@ def change
       t.index :created_at
     end
 
-    create_table :rmp_traces do |t|
+    create_table :rmp_traces, charset: 'utf8' do |t|
       t.belongs_to :rmp_profiled_request, null: false, foreign_key: true
       t.string :name
       t.bigint :start
@@ -36,7 +36,7 @@ def change
       t.timestamps
     end
 
-    create_table :rmp_flamegraphs do |t|
+    create_table :rmp_flamegraphs, charset: 'utf8' do |t|
       t.belongs_to :rmp_profiled_request, null: false, foreign_key: true
       t.binary :data