load session with bing-layer -> high zoom doesnt work anymore like in older version

[MKnight]

Steps to reproduce:

• Load a session which included a bing layer
• zoom in hard

Result:
tiles are not shown (only empty tiles with the camera symbol)

Steps to compare:
Variant 1:

• take older josm

Variant 2:

• dont load a session
• Background-imagery-> Bing-sat
• zoom in hard

Result:
a very subtle hint, imagery is stille shown (in last highest zoom)

german discussion about:
​http://forum.openstreetmap.org/viewtopic.php?id=31637