esteban-aliverti/guvnor — comment on commit 48291718ef1aae37716b414fa3c8bed2ad0a5983
File:  line 0
By esteban-aliverti:

According to HTTP 1.1 protocol (http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html) the Last-Modified header field is of type "HTTP-date"
The same protocol defines "HTTP-date" as "rfc1123-date | rfc850-date | asctime-date": http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html
And:
rfc1123-date = wkday "," SP date1 SP time SP "GMT"
rfc850-date = weekday "," SP date2 SP time SP "GMT"
asctime-date = wkday SP date3 SP time SP 4DIGIT
wkday = "Mon" | "Tue" | "Wed" | "Thu" | "Fri" | "Sat" | "Sun"
weekday = "Monday" | "Tuesday" | "Wednesday" | "Thursday" | "Friday" | "Saturday" | "Sunday"

So, I assumed that Locale.US must be used. Not sure if the protocol supports multiple locales or not.