Add retry after HTTP 502

[jpalmer]

A follow-on to #5920/r28893, I've now found an Esri server that sometimes returns a 502 error. e.g

ogr2ogr -f "PostgreSQL" "PG:dbname=test" "​http://maps.stats.govt.nz/wss/service/arcgis1/guest/Boundaries/2013_Geographies/MapServer/9/query?where=1=1&text=&objectIds=&time=&geometry=&geometryType=esriGeometryEnvelope&inSR=&spatialRel=esriSpatialRelIntersects&relationParam=&outFields=&maxAllowableOffset=&outSR=&outSR=&outFields=&outFields=&geometry=&returnZ=true&returnM=true&gdbVersion=&f=json" -nln ta_bdys

ERROR 1: HTTP error code : 502

This patch adds 502 to the list of error codes when a retry will be attempted.

[Even Rouault]

trunk r29037 "CPLHttpFetch(): add 502 to HTTP error codes that can be retried (#5942, patch by jpalmer)"

[jpalmer]

Thanks Even!