support BasicObject instance's as value in Pry#inject_local

before:
[6] pry(main)> _pry_.inject_local :foo, BasicObject.new, Pry.toplevel_binding
NoMethodError: undefined method `is_a?' for #<BasicObject:0x007fbb49f1a048>
from /Users/giorgio/code/pry/lib/pry/pry_instance.rb:151:in `inject_local'
[7] pry(main)> exit

after:
[3] pry(main)> _pry_.inject_local :foo, BasicObject.new, Pry.toplevel_binding
=> #<BasicObject:0x3fc186707a4c>
[4] pry(main)> foo
=> #<BasicObject:0x3fc186707a4c>