Fix corrupt output when adding to a dotted key inside an inline table

[gaoflow]

Adding a key to a dotted-key table inside an inline table produced unparseable output (#495):

doc = parse('a = {b.c = 1}\n')
doc["a"]["b"]["d"] = 2
doc.as_string()  # 'a = {b.c = 1d = 2\n}'

InlineTable.as_string rendered the dotted prefix once and then dumped the materialized table's block-style body verbatim. The fix renders one prefix.child = value pair per child (recursing for nested dotted keys), mirroring what Container._render_table does for dotted keys in regular tables, so the result is a = {b.c = 1, b.d = 2}. Repeating the prefix is required for correctness: rendering the new key bare ({b.c = 1, d = 2}) would change its meaning to a.d.

Fixes #495.