Reset ascii_only cache when calling String#replace
Reset ascii_only cache when calling String#replace

Methods such as IO#read rely on String#replace when reading into a String
buffer. Previously this would not clear the ascii_only cache, causing issues
such as those described in the issues below.

Fixes #3236
Fixes #3234

mlarraz/rubinius,ngpestelos/rubinius,ngpestelos/rubinius,kachick/rubinius,dblock/rubinius,ruipserra/rubinius,jsyeo/rubinius,Wirachmat/rubinius,ruipserra/rubinius,benlovell/rubinius,Azizou/rubinius,ngpestelos/rubinius,sferik/rubinius,benlovell/rubinius,mlarraz/rubinius,Azizou/rubinius,sferik/rubinius,Wirachmat/rubinius,kachick/rubinius,ruipserra/rubinius,jemc/rubinius,benlovell/rubinius,sferik/rubinius,heftig/rubinius,heftig/rubinius,Wirachmat/rubinius,kachick/rubinius,ngpestelos/rubinius,Azizou/rubinius,dblock/rubinius,sferik/rubinius,jsyeo/rubinius,digitalextremist/rubinius,sferik/rubinius,kachick/rubinius,Wirachmat/rubinius,ruipserra/rubinius,jsyeo/rubinius,mlarraz/rubinius,jemc/rubinius,ruipserra/rubinius,kachick/rubinius,ngpestelos/rubinius,heftig/rubinius,jsyeo/rubinius,kachick/rubinius,jsyeo/rubinius,heftig/rubinius,digitalextremist/rubinius,jemc/rubinius,Azizou/rubinius,Azizou/rubinius,sferik/rubinius,heftig/rubinius,ngpestelos/rubinius,Azizou/rubinius,digitalextremist/rubinius,benlovell/rubinius,kachick/rubinius,heftig/rubinius,dblock/rubinius,jsyeo/rubinius,jsyeo/rubinius,benlovell/rubinius,sferik/rubinius,dblock/rubinius,jemc/rubinius,mlarraz/rubinius,jemc/rubinius,dblock/rubinius,benlovell/rubinius,benlovell/rubinius,digitalextremist/rubinius,digitalextremist/rubinius,digitalextremist/rubinius,digitalextremist/rubinius,mlarraz/rubinius,Azizou/rubinius,kachick/rubinius,jemc/rubinius,mlarraz/rubinius,ruipserra/rubinius,Wirachmat/rubinius,Wirachmat/rubinius,ngpestelos/rubinius,mlarraz/rubinius,heftig/rubinius,dblock/rubinius,Wirachmat/rubinius,ruipserra/rubinius,dblock/rubinius,jemc/rubinius