Allow for explicit default loop creation via `get_hub(default=True)` after...

Allow for explicit default loop creation via `get_hub(default=True)` after default loop destruction. Include unit test.

gevent/core.ppyx

@@ -232,7 +232,7 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
     cdef libev.ev_timer _periodic_signal_checker
 #endif
 
-    def __init__(self, object flags=None, object default=True, size_t ptr=0):
+    def __init__(self, object flags=None, object default=None, size_t ptr=0):
         cdef unsigned int c_flags
         cdef object old_handler = None
         libev.ev_prepare_init(&self._signal_checker, <void*>gevent_run_callbacks)
@@ -245,8 +245,10 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
             c_flags = _flags_to_int(flags)
             _check_flags(c_flags)
             c_flags |= libev.EVFLAG_NOENV
-            if _default_loop_destroyed:
-                default = False
+            if default is None:
+                default = True
+                if _default_loop_destroyed:
+                    default = False
             if default:
                 self._ptr = libev.gevent_ev_default_loop(c_flags)
                 if not self._ptr:

gevent/hub.py

@@ -267,8 +267,8 @@ class Hub(greenlet):
                 raise TypeError("Unexpected argument: default")
             self.loop = loop
         else:
-            if default is None:
-                default = get_ident() == MAIN_THREAD
+            if default is None and get_ident() != MAIN_THREAD:
+                default = False
             loop_class = _import(self.loop_class)
             if loop is None:
                 loop = self.backend

greentest/test__destroy.py

 import gevent
+# Loop of initial Hub is default loop.
 hub = gevent.get_hub()
 assert hub.loop.default, hub
-hub.destroy()
 
+# Destroy hub. Does not destroy default loop if not explicitly told to.
+hub.destroy()
 hub = gevent.get_hub()
 assert hub.loop.default, hub
+
+# Destroy hub including default loop.
 hub.destroy(destroy_loop=True)
+# Create new hub and explicitly request creation of a new default loop.
+hub = gevent.get_hub(default=True)
+assert hub.loop.default, hub
 
+# Destroy hub including default loop.
+hub.destroy(destroy_loop=True)
+# Create new non-default loop in new hub.
 hub = gevent.get_hub()
 assert not hub.loop.default, hub
 hub.destroy()