Fix exhaustive case warnings

Starting with Crystal 0.34.0 case expressions issue a warning when
exhaustiveness can't be proven. This adds the missing else branches to
encode the default behaviour.

src/pg/escape_helper.cr

@@ -51,6 +51,8 @@ module PG
           num_backslashes += 1
         when quote_char
           num_quotes += 1
+        else
+          # skip
         end
       end
 
@@ -76,6 +78,8 @@ module PG
               build << quote_char
             when '\\'
               build << '\\' unless as_ident
+            else
+              # ignore
             end
             build << c
           end

src/pq/connection.cr

@@ -224,6 +224,8 @@ module PQ
           raise ConnectionError.new(
             "Only on is supported for integer_datetimes, got: #{frame.value.inspect}")
         end
+      else
+        # ignore
       end
     end
 

src/pq/conninfo.cr

@@ -94,6 +94,8 @@ module PQ
         @sslkey = value
       when "sslrootcert"
         @sslrootcert = value
+      else
+        # ignore
       end
     end
 

src/pq/frame.cr

@@ -19,6 +19,7 @@ module PQ
           when 'S' then ParameterStatus
           when 'K' then BackendKeyData
           when 'R' then Authentication
+          else          nil
           end
       k ? k.new(bytes) : Unknown.new(type, bytes)
     end

src/pq/notice.cr

@@ -19,6 +19,8 @@ module PQ
           code = f.message
         when :message
           message = f.message
+        else
+          # ignore
         end
       end