Make postgres adapter work with postgres-pr 0.7.0 (Fixes #1074)

This fixes an ArgumentError when trying to use Sequel with postgres-pr
0.7.0, because that started shipping with a pg.rb file.

Note that postgres-pr 0.7.0 doesn't work on ruby 2.0+ due to
multibyte escape issues in a regexp.  For years I've only been
testing using my postgres-pr fork.  Mention that fork in the doc
so that people are aware of it.

CHANGELOG

@@ -1,3 +1,7 @@
+=== HEAD
+
+* Make postgres adapter work with postgres-pr 0.7.0 (jeremyevans) (#1074)
+
 === 4.26.0 (2015-09-01)
 
 * Make Dataset#== not consider frozen status in determining equality (jeremyevans)

doc/opening_databases.rdoc

@@ -309,7 +309,9 @@ The Sequel postgres adapter works with the pg, postgres, and postgres-pr ruby li
 The pg library is the best supported, as it supports real bound variables and prepared statements.
 If the pg library is being used, Sequel will also attempt to load the sequel_pg library, which is
 a C extension that optimizes performance when Sequel is used with pg.  All users of Sequel who
-use pg are encouraged to install sequel_pg.
+use pg are encouraged to install sequel_pg.  For users who want to use postgres-pr to avoid issues
+with C extensions, it is recommended to use jeremyevans-postgres-pr, which fixes many issues in
+the upstream postgres-pr gem, and is regularly tested with Sequel.
 
 The following additional options are supported:
 

lib/sequel/adapters/postgres.rb

@@ -2,6 +2,10 @@
 
 begin 
   require 'pg' 
+
+  # Work around postgres-pr 0.7.0+ which ships with a pg.rb file
+  raise LoadError unless defined?(PGconn::CONNECTION_OK)
+
   SEQUEL_POSTGRES_USES_PG = true
 rescue LoadError => e 
   SEQUEL_POSTGRES_USES_PG = false