Support overwriting existing headers & response exception updates

[Ndiritu]

Changes:

• Use array_merge to allow customers to overwrite our default header values. Appending values will be at the discretion of the developer
• Support string response payloads from the Graph for 4xx/5xx errors. We had only anticipated JSON payloads from the Graph

Some breaking changes here:

• GraphResponseException takes in a StreamInterface as opposed to an array
• getRawResponseBody() return type changes from array to StreamInterface

Closes #53
Closes #54

[Ndiritu]

@MIchaelMainer @SilasKenneth just confirming that the versioning is ok for breaking changes in a preview release?

[MIchaelMainer]

We should also start an upgrade document to capture what changes are breaking. We will link to it in release notes and blog post announcements.

[MIchaelMainer]

The version change is fine since it is still RC for a major version change..

[Ndiritu]

The version change is fine since it is still RC for a major version change..

Should we update the existing v1 & beta RC's to use versions of core <= 2.0.0-RC2 & release new RC's for v1, beta that use this new core release?
Trying to think of a way of reducing the blast radius of the breaking changes

[Ndiritu]

We should also start an upgrade document to capture what changes are breaking. We will link to it in release notes and blog post announcements.

Can we leverage this guide and add a section at the top for upgrading between the current RC versions?

[MIchaelMainer]

Should we update the existing v1 & beta RC's to use versions of core <= 2.0.0-RC2 & release new RC's for v1, beta that use this new core release? Trying to think of a way of reducing the blast radius of the breaking changes

Yes, the sooner the better.

Can we leverage this guide and add a section at the top for upgrading between the current RC versions?

Yes, good idea.

[MIchaelMainer]

Once the change to upgrade guide is added we should be good here.

[Ndiritu]

Once the change to upgrade guide is added we should be good here.

@MIchaelMainer Please see the related PR's on v1 and beta