Fix headers for list of dicts

[jhadida]

This contribution corrects the behavior of the headers option when the input data is a list of dict.

1. Fix the behavior of headers="firstrow". The example below illustrates the current behavior, which is incorrect:

x = [ {'b': 'B', 'a': 'A'}, 
      {'a': 1, 'b': 2}, 
      {'a': 3, 'b': 4, 'c': 5} ]

print(tabulate( x, headers='firstrow' ))

# Current output:
#
#   B    A    c
# ---  ---  ---
#   2    1
#   4    3    5

# Proposed output:
#
#   B    A
# ---  ---
#   2    1
#   4    3

2. Fix the current behavior when headers is specified as a list of keys:

x = [ {'a': 1, 'b': 2}, 
      {'a': 3, 'b': 4, 'c': 5} ]

print(tabulate( x, headers=['b','a'] ))

# Current output:
#
#       b    a
# --  ---  ---
#  1    2
#  3    4    5

# Proposed output:
#
#   b    a
# ---  ---
#   2    1
#   4    3