Guard for common case of String#<< receiving a string
Guard for common case of String#<< receiving a string

When we give String#<< a String as argument, before we were doing two
type checks. By reordering them, we only have to do 1 in the common
case.

sferik/rubinius,ruipserra/rubinius,heftig/rubinius,kachick/rubinius,ngpestelos/rubinius,dblock/rubinius,heftig/rubinius,Azizou/rubinius,benlovell/rubinius,heftig/rubinius,Wirachmat/rubinius,pH14/rubinius,digitalextremist/rubinius,heftig/rubinius,dblock/rubinius,mlarraz/rubinius,kachick/rubinius,Azizou/rubinius,digitalextremist/rubinius,ruipserra/rubinius,mlarraz/rubinius,jemc/rubinius,kachick/rubinius,sferik/rubinius,sferik/rubinius,digitalextremist/rubinius,Wirachmat/rubinius,heftig/rubinius,jsyeo/rubinius,mlarraz/rubinius,benlovell/rubinius,digitalextremist/rubinius,jemc/rubinius,mlarraz/rubinius,dblock/rubinius,ngpestelos/rubinius,jsyeo/rubinius,kachick/rubinius,benlovell/rubinius,Azizou/rubinius,benlovell/rubinius,Azizou/rubinius,ruipserra/rubinius,jemc/rubinius,ngpestelos/rubinius,pH14/rubinius,ngpestelos/rubinius,mlarraz/rubinius,sferik/rubinius,ruipserra/rubinius,digitalextremist/rubinius,pH14/rubinius,ngpestelos/rubinius,ruipserra/rubinius,jemc/rubinius,jsyeo/rubinius,sferik/rubinius,heftig/rubinius,dblock/rubinius,Wirachmat/rubinius,Wirachmat/rubinius,benlovell/rubinius,jsyeo/rubinius,ruipserra/rubinius,dblock/rubinius,ngpestelos/rubinius,kachick/rubinius,sferik/rubinius,benlovell/rubinius,pH14/rubinius,Azizou/rubinius,Azizou/rubinius,digitalextremist/rubinius,pH14/rubinius,ngpestelos/rubinius,kachick/rubinius,sferik/rubinius,Wirachmat/rubinius,pH14/rubinius,dblock/rubinius,jemc/rubinius,ruipserra/rubinius,Wirachmat/rubinius,jsyeo/rubinius,jsyeo/rubinius,kachick/rubinius,jemc/rubinius,Azizou/rubinius,jemc/rubinius,mlarraz/rubinius,dblock/rubinius,mlarraz/rubinius,jsyeo/rubinius,digitalextremist/rubinius,benlovell/rubinius,heftig/rubinius,kachick/rubinius,Wirachmat/rubinius,pH14/rubinius