deserialize_header function needs to be refactored

[mattsb42-aws]

As pylint and flake8 justifiably point out, internal.formatting.deserialize.deserialize_header is too complex. Refactor to simplify.

186590df9307:aws-encryption-sdk-python bullocm$ tox -e flake8,pylint
GLOB sdist-make: /Users/bullocm/git/aws-encryption-sdk-python/setup.py
flake8 inst-nodeps: /Users/bullocm/git/aws-encryption-sdk-python/.tox/dist/aws-encryption-sdk-1.3.2.zip
flake8 installed: asn1crypto==0.23.0,attrs==17.2.0,aws-encryption-sdk==1.3.2,boto3==1.4.7,botocore==1.7.17,cffi==1.11.0,cryptography==2.0.3,docutils==0.14,flake8==3.4.1,flake8-docstrings==1.1.0,flake8-import-order==0.13,flake8-polyfill==1.0.1,idna==2.6,jmespath==0.9.3,mccabe==0.6.1,pycodestyle==2.3.1,pycparser==2.18,pydocstyle==2.0.0,pyflakes==1.5.0,python-dateutil==2.6.1,s3transfer==0.1.11,six==1.11.0,snowballstemmer==1.2.1,wrapt==1.10.11
flake8 runtests: PYTHONHASHSEED='1582386464'
flake8 runtests: commands[0] | flake8 src/aws_encryption_sdk/ setup.py
src/aws_encryption_sdk/internal/formatting/deserialize.py:63:1: C901 'deserialize_header' is too complex (15)
ERROR: InvocationError: '/Users/bullocm/git/aws-encryption-sdk-python/.tox/flake8/bin/flake8 src/aws_encryption_sdk/ setup.py'
pylint inst-nodeps: /Users/bullocm/git/aws-encryption-sdk-python/.tox/dist/aws-encryption-sdk-1.3.2.zip
pylint installed: asn1crypto==0.23.0,astroid==1.5.3,attrs==17.2.0,aws-encryption-sdk==1.3.2,boto3==1.4.7,botocore==1.7.17,cffi==1.11.0,coverage==4.4.1,cryptography==2.0.3,docutils==0.14,idna==2.6,isort==4.2.15,jmespath==0.9.3,lazy-object-proxy==1.3.1,mccabe==0.6.1,mock==2.0.0,pbr==3.1.1,py==1.4.34,pycparser==2.18,pyflakes==1.6.0,pylint==1.7.2,pytest==3.2.2,pytest-catchlog==1.2.2,pytest-cov==2.5.1,pytest-mock==1.6.3,python-dateutil==2.6.1,s3transfer==0.1.11,six==1.11.0,wrapt==1.10.11
pylint runtests: PYTHONHASHSEED='1582386464'
pylint runtests: commands[0] | pylint --rcfile=pylintrc src/aws_encryption_sdk/ setup.py
************* Module aws_encryption_sdk.internal.formatting.deserialize
src/aws_encryption_sdk/internal/formatting/deserialize.py:63: [R0914(too-many-locals), deserialize_header] Too many local variables (27/15)
src/aws_encryption_sdk/internal/formatting/deserialize.py:63: [R0915(too-many-statements), deserialize_header] Too many statements (54/50)

------------------------------------------------------------------
Your code has been rated at 9.99/10 (previous run: 9.99/10, +0.00)

ERROR: InvocationError: '/Users/bullocm/git/aws-encryption-sdk-python/.tox/pylint/bin/pylint --rcfile=pylintrc src/aws_encryption_sdk/ setup.py'
__________________________________________________________________________________________________ summary ___________________________________________________________________________________________________
ERROR:   flake8: commands failed
ERROR:   pylint: commands failed