SequenceOf/SetOf to remain a schema objects (#162)

* Add `omitEmptyOptionals` encoder option

Added `omitEmptyOptionals` option which is respected by `Sequence`
and `Set` encoders. When `omitEmptyOptionals` is set to `True`, empty
initialized optional components are not encoded. Default is `False`.

* Change `SequenceOf`/`SetOf` behaviour

- New elements to `SequenceOf`/`SetOf` objects can now be added at any
  position - the requirement for the new elements to reside at the end
  of the existing ones (i.e. s[len(s)] = 123) is removed.

- Removed default initializer from `SequenceOf`/`SetOf` types to ensure
  consistent behaviour with the rest of ASN.1 types. Before this change,
  `SequenceOf`/`SetOf` instances immediately become value objects
  behaving like an empty list. With this change, `SequenceOf`/`SetOf`
  objects remain schema objects unless a component is added or
  `.clear()` is called.

- Added `.reset()` method to all constructed types to turn value object
  into a schema object.

2 files changed, 4 insertions, 0 deletions

diff --git a/tests/codec/ber/test_encoder.py b/tests/codec/ber/test_encoder.py
index 26819bd..da14830 100644
--- a/tests/codec/ber/test_encoder.py
+++ b/tests/codec/ber/test_encoder.py
@@ -476,6 +476,7 @@ class UTF8StringEncoderWithSchemaTestCase(BaseTestCase):
 class SequenceOfEncoderTestCase(BaseTestCase):
     def testEmpty(self):
         s = univ.SequenceOf()
+        s.clear()
         assert encoder.encode(s) == ints2octs((48, 0))
 
     def testDefMode(self):
@@ -570,6 +571,7 @@ class SequenceOfEncoderWithComponentsSchemaTestCase(BaseTestCase):
 class SetOfEncoderTestCase(BaseTestCase):
     def testEmpty(self):
         s = univ.SetOf()
+        s.clear()
         assert encoder.encode(s) == ints2octs((49, 0))
 
     def testDefMode(self):
diff --git a/tests/codec/cer/test_encoder.py b/tests/codec/cer/test_encoder.py
index d9d9212..ea8f813 100644
--- a/tests/codec/cer/test_encoder.py
+++ b/tests/codec/cer/test_encoder.py
@@ -155,6 +155,7 @@ class UTCTimeEncoderTestCase(BaseTestCase):
 class SequenceOfEncoderTestCase(BaseTestCase):
     def testEmpty(self):
         s = univ.SequenceOf()
+        s.clear()
         assert encoder.encode(s) == ints2octs((48, 128, 0, 0))
 
     def testDefMode1(self):
@@ -219,6 +220,7 @@ class SequenceOfEncoderWithSchemaTestCase(BaseTestCase):
 class SetOfEncoderTestCase(BaseTestCase):
     def testEmpty(self):
         s = univ.SetOf()
+        s.clear()
         assert encoder.encode(s) == ints2octs((49, 128, 0, 0))
 
     def testDefMode1(self):