VariantContextBuilder : putAttributes fails if Map cannot be modified

[lindenb]

Subject of the issue

I got an exception with VariantContextBuilder.putAttributes: UnsupportedOperationException

in https://github.com/samtools/htsjdk/blob/master/src/main/java/htsjdk/variant/variantcontext/VariantContextBuilder.java#L273

if this.attributes is not null and was created from public VariantContextBuilder(final VariantContext parent) , as far as I can see this.attributes is unmodifieable.

I could write a PR but I don't really understand this attributesCanBeModified.

Your environment

• version of htsjdk 2.2.4
• version of java 1.8
• which OS linux

[yfarjoun]

Thanks @lindenb. I think that this might be related to #1344. I'm adding your usecase as a test to check it. be happy to get your review on that PR if you're interested.